1. Field of Invention
Embodiments of the present invention generally relate to processes for producing films from compositions containing blends of polyethylenes, particularly processes for producing films from compositions containing blends of bimodal polyethylene and unimodal polyethylene, and film resin compositions.
2. Description of Related Art
Polyethylene compositions have been used to make films, including bimodal polyethylene compositions. Bimodal polyethylene compositions have numerous advantages over unimodal polyethylene compositions, and have solved various problems in the art. High molecular weight bimodal high density polyethylene compositions have been used to make film as well as pipes. Unlike pipe resins, a film resin has to satisfy several critical processing attributes: bubble stability, draw-down capability, and film quality. Many good bimodal pipe resins do not necessarily serve as good film resins, which require the above mentioned film processing attributes. In some cases, certain bimodal pipe resins exhibit poor bubble stability and poor film quality, when they are film extruded, with undesirable film texture where many hazy lines are dispersed all over the film surface, and therefore are unsatisfactory for film applications.
The undesirable film texture of hazy lines dispersed on the film surface is believed to stem from inhomogeneous molten polymer. When the bimodal polyethylene, which consists of high molecular weight component and low molecular weight component, is extruded from an annular die and expanded to form a blown bubble, crystallization takes place near the frost line. If the two components of the bimodal polyethylene are not well homogenized in a molten state, either due to their huge disparity in molecular weight or in viscosity, the respective component crystallizes separately forming discrete phases in a solid state. This results in poor film texture with hazy lines dispersed all over the film surface.
The homogeneity of bimodal polyethylene is believed to be dependent on the degree of spread, which describes the bimodality of the bimodal polyethylene, i.e., the relative distance between the two molecular weight peaks of component polymers, which can be seen from a size exclusion chromatography. It appears to be an optimum range of spread that prevents a blown film of bimodal polyethylene from being resulted in poor film texture.
Often, bimodal polyethylenes of poor film texture are accompanied with poor bubble stability. The inhomogeneity of the molten polymer leads to poor melt strength, which results in a limited capability of film blowability, i.e., poor bubble stability and, poor draw-down capability.
An ongoing need exists for further understanding on the film blowability of polyethylene resin, particularly for bimodal polyethylene compositions used to make films. This invention discusses polyethylene compositions that comprise of bimodal polyethylene having a wider spread and unimodal polyethylene.
Earlier patents and published applications describe various aspects of polyethylene compositions that contain mixtures of different types of polyethylenes, including Sakurai et al., U.S. Pat. No. 4,336,352 which involves blends of different ethylene polymers; Whaley, U.S. Pat. No. 6,359,072, which involves films formed from blends of polyethylene resins; Debras et al., U.S. Pat. No. 6,566,450, relates to multimodal polyethylenes; Laurent, U.S. Pat. No. 6,579,922 involves pipe resin blends that include bimodal polyethylene; McDaniel et al., U.S. Pat. No. 6,525,148, relates to catalyst systems for making polyethylenes; Clutton et al., U.S. Pat. No. 6,489,427, relates to polymer compositions; McDaniel et al., U.S. Pat. No. 6,388,017, relates to various polymer compositions; de Lange et al., U.S. Pat. No. 6,545,093, relates to bimodal polyethylene blends; Nummila-Parkarinen et al., U.S. Pat. No. 6,562,905, relates to high density polyethylene compositions; Promel, U.S. Pat. No. 6,344,522, relates to ethylene polymer compositions; Jacobsen et al., U.S. Pat. No. 6,506,866 relates to ethylene copolymer compositions; Vandun et al., PCT Application WO 03/016396, relates to bimodal polyethylene compositions; Mattioli et al., PCT Application WO 02/102891 relates to ethylene polymer compositions; Ahistrand, EP 1 146 078, relates to polymer compositions for pipes; and Laurent, EP 1 041 113, relates to polyolefins.
Also, patents that refer to films and/or polyethylene compositions, as well as methods for making polyethylene, include the following: U.S. Pat. Nos. 4,336,352; 5,091,228; 5,110,685; 5,208,309; 5,274,056; 5,635,262; 5,338,589; 5,344,884; 5,378,764; 5,494,965; 5,739,225; 5,795,941; 6,090,893; 6,340,730; 6,359,072; 6,388,017; 6,388,115; 6,403,717; 6,420,580; 6,441,096; 6,476,166; 6,534,604; 6,562,905; 6,605,675; 6,608,149; and WO 97/47682 and WO 94/22948. Other patents and publications are listed on the cover page of the patent.